In many scientific applications, it is necessary to focus an electron signal. For example, in a mass spectrometer the analyte is ionized to form a range of charged species. The resultant ions are then separated according to their mass-to-charge ratio, typically by acceleration and exposure to an electric or magnetic field. The separated ions impact on an ion detector to generate a signal. Results are displayed as a spectrum of the relative abundance of detected ions as a function of the mass-to-charge ratio.
The impact of an input ion on the impact surface of a detector may be amplified in some manner, typically by an electron multiplier. Generally the impact surface is incorporated within the electron multiplier. The electron multiplier may operate by way of secondary electron emission whereby the impact of a single or multiple ion(s) on the multiplier surface causes single or multiple electrons associated with atoms of the surface to be released. It is these secondary electrons which form the principle signal to be amplified by the detector.
It is generally desirable for an electron multiplier to have a large sensitive input area so that particles can be detected which are incident over a large area. This requirement often results in a mismatch between the desired sensitive input area and the sensitive area of the amplifying section of the electron multiplier (which is generally significantly smaller). In these circumstances it is desirable to include a focussing element, often referred to as a focussing lens, between the device's input aperture and its amplifying section. The focussed electrons typically impact a target electrode that is entrant to the electron multiplier's amplifying section.
There are many different types of electron multipliers known in the art and for each type there is usually a preferred or optimal manner for focussing electrons. For example, a continuous channel electron multiplier typically utilizes a resistive cone; while an electrostatic discrete dynode electron multiplier often utilizes purely electrostatic focussing. In some electron multipliers the need for focussing is completely obviated, an example being micro channel plate (MCP) electron multipliers.
One type of electron multiplier controls the path of secondary electrons from the emission surface to the target surface by the use of crossed magnetic and electrostatic fields. An example of this form of multipliers disclosed in United States Patent published as U.S. Pat. No. 6,982,428 B2 (to STRESAU et al). These electrostatic/magnetic cross field electron multipliers have significant advantages over alternative electron multipliers. For example, such multipliers display minimal time distortion in electron transit times from the emission surface to the target surface.
A problem of some electrostatic/magnetic cross field electron multipliers of the prior art is that different areas of the ion impact surface exhibit different gains. In some configurations electrons emitted from the impact surface in a region distal to the target electrode undergo several more electron impact cycles than electrons from a region proximal to the target electrode. Each impact cycle results in additional emitted secondary electrons. As a result, a growing number of electrons accumulate across the impact surface. A detector utilizing this arrangement has a very large variation in gain from one end to the other of its ion impact surface and hence its ion input aperture and in effect a significantly reduced effective sensitive area or a skewed response across its sensitive area.
This problem with variable gain might be overcome by arranging the electron impact energy such that the average secondary electron yield (i.e. the average number of secondary electrons emitted from a single electron impact) is close to 1. This solves the gain variation problem but introduces yet a further problem. Because secondary electron emission follows a Poisson distribution for the probability of an emission, using a low average yield leads to a significant probability of no emission at all. An electron impact with an average secondary electron yield of 1.0 will have a 37% probability of zero emission. The net result will be an ion detection efficiency of 63% for the first interaction with a corresponding diminishing efficiency for each successive electron-surface interaction. Clearly, this solution is unacceptable for many applications.
A method for electron focussing was addressed by the inventors of U.S. Pat. No. 6,982,428 by utilizing a reduction in the E/B2 ratio (ratio of the electrostatic field to the square of the magnetic field) along the path of electron flow to accomplish the required focussing. A disadvantage of this system is the need for a large and complex magnetic circuit.
A further problem of the art is that secondary electrons may be reabsorbed into the impact surface. As a result the detector sensitivity progressively diminishes with increased distance from the target end of the impact surface. As discussed supra, this results in the detector having a very large variation in gain from one end to the other of its ion impact surface and in effect a significantly reduced effective sensitive area or a skewed response across its sensitive area.
It is an aspect of the present invention to provide improved means for focussing electrons, and particularly secondary electrons emitted from the impact plate of an electron multiplier. It is a further aspect to provide a useful alternative to prior art means for focussing electrons.
The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.